CODEWITHSUNDEEP
c++cshared-librariesdynamic-linking
user7953556
user7953556

Reputation: 47

Error with shared library as "invalid conversion" from void * to double(*) (int*)?

Hi i am trying to implement shared library (dynamic linking) below is the code i am getting error as below please help me to fix it 

error: invalid conversion from ‘void*’ to ‘double (*)(int*)’ [-fpermissive]
  fn = dlsym(lib_handle, "ctest1");

ctest1.c

void ctest1(int *i)
{
   *i=5;
 }

Above ctest1.c is the shared library which is used in the below hello.cc file

#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
#include "ctest1.h"  // here i have declared the function of shared library

int main(int argc, char **argv)
{
   void *lib_handle;
   void (*fn)(int *);
   int x=990;
   char *error;

   lib_handle = dlopen("libp.so", RTLD_LAZY);  // opening the shared library
   if (!lib_handle)
   {
      fprintf(stderr, "%s\n", dlerror());
      exit(1);
   }

 fn = dlsym(lib_handle, "ctest1");  //storing the address of shared library function 
   if ((error = dlerror()) != NULL)
   {
      fprintf(stderr, "%s\n", error);
      exit(1);
   }

   fn(&x);
   printf("getting x value from shared library=%d\n",x);

   dlclose(lib_handle);
   return 0;
}

~
~
~

Upvotes: 0

Views: 1133

Answers (2)

Iharob Al Asimi
Iharob Al Asimi

Reputation: 53016

You are simply invoking the wrong compiler. In you can't convert from void * to another pointer type without casting. If this is not your code then, the lack of a cast means that the code is instead of . Please read the tags wiki to understand, c and c++ are not the same language, they are somewhat similar but, certainly not the same.

This is from draft n1570 of the standard

6.3.2.3 Pointers

  1. A pointer to void may be converted to or from a pointer to any object type. A pointer to any object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer.

If it's your code you should distinguish between c and c++ by using the appropriate file extension, or force the compiler to use the appropriate compiler which I don't recommend, just fix the file extension.

Upvotes: 3

Sami Kuhmonen
Sami Kuhmonen

Reputation: 31173

dlsym returns a void* and you are trying to store that in the function pointer. You have to use a cast for it to succeed when using c++ (assuming from tags and file ending .cc, though your code is actually C).

Also your type claims the function has a return type of double but your library function returns void. This will have to be fixed or you will get runtime problems.

Upvotes: 1

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